1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus.
2. Description of the Background Art
Generally, in an electrophotographic image forming apparatus, moisture, nitric compounds, ammonia gas and other substances harmful to image formation bring about the following various problems if present around a photoconductive element.
As for moisture, charged fine grains, e.g., toner and carrier grains have their electrostatic charging conditions stabilized with a charge control agent added to high-molecular resin. The high-molecular resin, however, adsorbs moisture present in air even if subjected to processing for hydrophobicity, resulting in the variation of electric resistance, frictional resistance between the grains and fluidity. Consequently, the amount of charge to deposit on the toner grains decreases and, in turn, increases image density or otherwise effects image quality.
Nitric compounds present in air around a photoconductive element are coupled with moisture also present in air to produce nitric acid, nitrate and other ionized substances and cause them to deposit on the photoconductive element. The ionized substances accelerate the deterioration of the surface of the photoconductive element and thereby cause the surface to wear at an unexpected rate. Further, the ionized substances render the surface of the photoconductive element electrically conductive with the result that a latent image formed on the surface is blurred. Nitric compounds, which are derived from discharge for charging the surface of the photoconductive element, exist in a great amount around a charger, i.e., the image forming apparatus.
Ammonia gas present in air produces ammonium nitrate when effected by the discharge of the charger. Ammonium nitrate causes the photoconductive element to sharply deteriorate. The amount of ammonia gas in air is great around a diazo copier using ammonia.
In light of the above, to enhance the durability of the photoconductive element and therefore image quality, it is necessary to remove or reduce moisture, nitric compounds and ammonia gas around the photoconductive element. Japanese Patent Laid-Open Publication No. 7-72770, for example, discloses an electrophotographic image forming apparatus configured to remove substances harmful to image formation around a photoconductive element. The apparatus disclosed in this document removes moisture in air around a photoconductive element, ozone produced by the discharge of a charger, and paper dust also present in air. For this purpose, in such an apparatus, an air conditioning mechanism sucks air from the entire space inside the apparatus while feeding conditioned air to the same space. This air conditioning scheme, however, must cover the entire space of the apparatus and therefore makes the air conditioning mechanism bulky and expensive.
Japanese Patent Laid-Open Publication No. 6-236132, for example, teaches an electrophotographic image forming apparatus in which a hygroscopic substance is positioned in the vicinity of a photoconductive element. The hygroscopic substance lowers absolute humidity around the photoconductive element to thereby prevent nitric compounds produced by the discharge of a charger from being coupled with moisture. This apparatus, like the apparatus of Laid-Open Publication No. 7-72770, dehumidifies the entire space inside the apparatus and cannot sufficiently remove moisture around the photoconductive element because moisture relatively freely flows into the apparatus.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Publication No. 6-82234 and Japanese Patent Laid-Open Publication Nos. 6-130773 and 2000-98855.
It is an object of the present invention to provide an image forming apparatus capable of efficiently removing substances harmful to image formation around a photoconductive element.
An image forming apparatus of the present invention includes a casing, an image forming unit, and an air conditioning device. The image forming unit includes a case accommodating at least a photoconductive element therein and formed with an opening for image transfer via which the photoconductive element is partly exposed to the outside. The opening is the only opening that might allow substances harmful to image formation into the case when the image forming unit is mounted to the casing. The air conditioning device removes the harmful substances flowing into the image forming unit from the outside.